The invention relates to a process for producing foamed polyolefin-based plastics blocks, in particular based on crosslinked polyethylene.
The production of crosslinked polyethylene foams generally starts from what is known as a matrix, a compact plastics sheet or plastics web, which comprises the foaming agent and, where appropriate, a crosslinking agent in finely divided form, but has not yet been foamed. The matrix is generally extruded at a temperature which must be below the decomposition or reaction temperature of the crosslinking agent and foaming agent.
In the crosslinking, a distinction is made between
physical crosslinking by means of high-energy radiation, such as xcex2- or xcex3-radiation
chemical crosslinking, e.g. by means of organic peroxides, which when a particular temperature is exceeded bring about crosslinking of the plastics.
In relation to the foaming, a distinction is made between
continuous foaming of a web product made from a continuous matrix whose thickness is up to about 5 mm, and
what is known as the block foaming process: batch foaming of relatively large sheets of matrix.
Whereas foam blocks produced by the block foaming process and continuously produced web product with physical crosslinking have very fine cells, continuous production of chemically crosslinked foams gives a rather coarser cell structure. Examples of typical cell sizes are from 0.1 to 0.2 mm for chemically crosslinked block foam, from 0.6 to 0.8 mm for chemically crosslinked foam from continuous foaming, and from 0.3 to 0.4 mm for physically crosslinked foam from continuous foaming.
A process for the continuous production of chemically crosslinked polyolefin-based foam webs is described by way of example in DE 16 94 130 C3, while the block foaming process with chemical crosslinking is known by way of example from DE 34 30 108 C2 and EP 0 186 110 B1. Finally, U.S. Pat. No. 3,562,367 A relates to a process for the continuous production of physically crosslinked foam webs by using electron beams (xcex2-radiation) for crosslinking.
Whereas the production of continuous polyethylene-based foam webs with either physical crosslinking or else with chemical crosslinking is widely practiced, only one chemical crosslinking method for producing block foam has been disclosed hitherto.
The crosslinking agent generally used is dicumyl peroxide. Since dicumyl peroxide is not without some physiological risk, precautionary reasons prevent the use of any relatively large amounts of dicumyl peroxide for crosslinking foams for products related to food or drink.
It was therefore an object of the present invention to provide a process which can produce crosslinked block foam based on polyolefins, in particular polyethylene, and which gives fine-celled foams while using no, or only very little, dicumyl peroxide.
The invention achieves this object by means of a process according to Claim 1, preferably in combination with one or more of the features of the subclaims.
A substantive feature here is the use of xcex2-radiation, i.e. of electron beams, where the energy of the electrons is  greater than 6 MeV, and where the thickness of the matrix is from 25 to 45 mm. It has been found that if electron beams of  less than 6 MeV are used the crosslinking of the matrix is inhomogeneous, giving the foam non-uniform quality and non-uniform physical properties.
Preference is given to xcex2-radiation whose energy is from 8 to 15 MeV, and the thickness of the matrix here is preferably from 30 to 40 mm.
The radiation dose is from 20 to 150 kJ/kg [= from 20 to 150 kGy], preferably from 40 to 70 kJ/kg. If the radiation dose is below 20 kJ/kg, the crosslinking of the matrix is inadequate, while if the dose is above 150 kJ/kg the crosslinking is excessive and impedes the foaming process.
The radiation dose can be set within the lower range from 20 to 80 kJ/kg by adding amounts of from 0.05 to 2% by weight, based on the matrix, of crosslinking aids, such as trimethylpropane trimethacrylate [sic], diallyl phthalate, allyl methacrylate or triallyl cyanurate, whereas if the material used has no crosslinking aids or crosslinking accelerators a somewhat higher xcex2-radiation dose is needed.
Using the high radiation dose according to the invention with very high-energy electrons leads to marked heating of the matrix through the irradiation. It is therefore preferable to carry out the crosslinking in a number of xcex2-radiation passes, with in each case a partial dose of from 10 to 50 kJ/kg. Between each two passes, it is preferable for the matrix to be cooled. When crosslinking in a number of passes it is particularly preferable for irradiation from the upper side to alternate with irradiation from the other side. This achieves uniform crosslinking of the matrix.
In another preferred embodiment of the invention for high radiation doses, the temperature in the matrix is controlled by adapting the level of the partial dose in an individual irradiation pass in such a way that the temperature of the matrix never exceeds 70xc2x0 C., the maximum temperature preferably being not more than 50xc2x0 C.
The use of high-energy electron beams causes a chemical alteration at the surface of the matrix, in particular an oxidation process, which causes skinning and surface cracking during subsequent foaming of the matrix. According to the invention, therefore, atmospheric oxygen is at least substantially prevented from reaching the matrix. This is preferably achieved by applying a paste or high-viscosity composition, for example a wax or the like. In an alternative preferred embodiment of the invention, an envelope made from a film of, for example, polyethylene at least substantially prevents atmospheric oxygen from reaching the matrix during the irradiation.
The thermoplastic used is a polyolefin composed at least predominantly of polyethylene and/or of ethylene copolymer. Particular preference is given here to LDPE, HDPE, LLDPE, VLDPE, a mixture of these polyethylene types, or a mixture of one or more of these polyethylene types with EVA and/or EPM or EPDM.
In another preferred embodiment of the invention, the matrix also comprises from 0.05 to 5% by weight of zinc oxide to lower the decomposition temperature of the blowing agent.
In another embodiment of the invention, the matrix preferably comprises from 0.05 to 1% by weight of fine-particle magnesium hydroxide. This reduces the cell size of the cells in the foam.
It has been found that the quality of the resultant foam is particularly good if a waiting time of at least 24 h is maintained between crosslinking by electron beam and foaming.
The invention is suitable for producing foam blocks whose average density is from 20 to 180 kg/m2 [sic], preferred densities being from 30 to 130 kg/m2 [sic].
The preferred foaming agent or blowing agent used is azodicarbonamidexe2x80x94as in the prior art. The foaming itself takes place in a manner known per se in enclosed, heated molds, the size of the mold being greater than the matrix by the desired foaming factor.